(1) Field of the Invention
This invention relates to the electrolytic recovery of solids from liquid media where at least a portion of the liquid media may be simultaneously filtered, most preferably by circulation through an ultrafiltration unit to maintain a relatively constant solids concentration in the electrolytic cell; and to the apparatus used to achieve the separation of the above-mentioned solids from liquid media.
(2) State of the Art
The separation of suspended solids from a liquid has historically been achieved by such well-known methods as filtration or centrifuging. There are, however, a great number of solid-liquid compositions where these simple techniques do not achieve the desired separation or no separation at all. Examples of such solid-liquid compositions are polymer latex and emulsion compositions, particularly compositions containing polymer particles of small particle size (less that 1 to 2 microns), colloidal suspensions, compositions where solid particles have a high affinity for the liquid or solvent or where these more simple techniques cause processing problems such as agglomeration. Several processes have been proposed to effect the separation of these "special" solid-liquid compositions. Among these are freeze-thaw techniques, extrusion drying and spray-drying. Of these, spray-drying is probably the most effective and most widely used technique, particularly with emulsion resins. The spray-drying process, however, requires substantial amounts of energy in order to raise the material to the liquid vaporization temperature and also to overcome the heat of vaporization of the liquid, e.g., water. In this present era of dwindling energy supplies, a less energy intensive recovery and drying process would be extremely desirable. Another problem encountered with techniques such as spray-drying is that there is commonly a broad particle size distribution range for the dried solid. While this may not be critical in many applications, size classification is important in some of the more refined applications of the dried solids usage such as in PVC plastisols. Hence, a solids recovery process which would result in particles of more uniform size would be extremely useful for specialized applications such as the above-mentioned plastisols which generally require particle sizes less than about 2 microns.
Electrophoretic techniques have been suggested for the recovery of suspended solids from liquid compositions to save energy, give better control of size classification and to provide a more economical recovery process. Electrophoretic techniques have long been used for depositing paint material coatings onto electrically conductive substrates, and more recently these techniques have been used in the electrolytic deposition of various solids on an anode. For example, British Pat. No. 1,525,103 discloses a method of concentrating polyvinyl chloride (PVC) emulsion which comprises the steps of electrolytically attracting the solids towards an anode which is covered with a porous nonconductive element so that the solids of PVC are deposited on the element and then removing the deposited solids from the porous element. In this process, PVC, as in electrolytic depositions, is acidic and has to be neutralized in a separate step and vessel after removal from the membrane. U.S. Pat. No. 3,664,938 illustrates the recovery of polymer fines, specifically polyacrylamide, by subjecting a water-in-oil suspension to an electrical field where the polymer fines deposit on an electrode and are removed. U.S. Pat. No. 4,146,455 teaches treating liquid whey by subjecting raw whey to forced flow electrophoresis to effect the separation of lactose and simultaneously concentrate the solids in the resulting product whey solution. U.S. Pat. No. 4,110,189 illustrates the electrokinetic separation of finely divided clay particles from an aqueous suspension thereof.
U.S. Pat. No. 3,449,227 illustrates the manufacture of asbestos articles by electrodepositing asbestos fibers from a continuously circulated dispersion onto an electrode. U.S. Pat. No. 3,436,326 disloses the removal of waste solids from an aqueous system by electroplating said solids on a positively charged screen which screen must be removed to be cleaned. U.S. Pat. No. 3,424,663 teaches the electrophoretic deposition of a synthetic resin onto a metallic substrate from an aqueous solution to produce an adherent coating. The disclosed process requires the presence of a complexing agent, such as EDTA, in the aqueous disperson.
The concentrating of polymer solids in an aqueous medium by ultrafiltration has been disclosed in the art in U.S. Pat. No. 3,956,114 and Japanese Kokai No. 18788/1977. The Japanese Patent further discloses that concentrating PVC solids by ultrafiltration requires further processing by spray-drying to obtain the desired separation of the solids from water.
Effecting the separation of suspended solids from a liquid by employing either electrolysis or ultrafiltration alone has not yielded the desired separation. The practical problem is that ultrafiltration cannot remove all the liquid, and electrolysis does not remove all the suspended solids alone. Thus, both processes must be followed by some sort of additional processing, e.g., coagulation, filtrations, spray-drying, extrusion drying, etc.
It is disclosed in U.S. Pat. Nos. 3,663,406 and 3,663,403 and Canadian Pat. No. 968,743 to use electrolysis in conjunction with ultrafiltration along with an additional processing step for electrodepositing coatings onto electrically conductive substrates where the function of the ultrafiltration step is to remove contaminates and generally to maintain stability in the electrodeposition bath. It is not found in the art where electrophoretic techniques are used in conjunction with ultrafiltration to recover suspended solids from a liquid medium.